1. Field of the Invention
This invention relates to an ignition timing control system for an internal combustion engine, more particularly to an ignition timing control system for a direct injection spark ignition engine, in which gasoline fuel is directly injected into the combustion chamber of the engine.
2. Description of the Related Art
In the control system of internal combustion engines, it is well-known to determine the fuel injection amount and the ignition timing based on the engine speed and engine load, for example, from Japanese Laid-Open Patent Application No. Hei 9 (1997)-4499. The system in this prior art has a dual-spark ignition coil which is connected to two cylinders whose operating cycles are 360.degree. out of phase with each other, in which it is determined whether the electric discharging period after ignition and the fuel injection period are overlapped with each other in the associated non-combustion cylinder (ignited in the exhaust stroke) and if the result is affirmative, the fuel injection start timing is delayed until the electric discharging has been finished.
Aside from the above, a direct injection spark ignition engine has recently been proposed in which gasoline fuel is directly injected into the combustion chamber such that an ultra-lean burn combustion or a stratified combustion (in an ultra lean air/fuel ratio) or the pre-mixture charged combustion (in a uniform air/fuel ratio) occurs in the engine as is disclosed in, for example, Japanese Patent Publication No. Hei 4 (1992) - 37264.
The ignition timing is normally set to a crank angular point at which the injected fuel has atomized (vaporized) and is combustible. However, in the direct injection spark ignition engine, since fuel injection is conducted in the compression stoke when the engine is operated at the ultra-lean burn combustion, the fuel injection end timing and the ignition timing are likely to overlap each other. This overlapping may lead to misfiring or to residual of unburned fuel such as HC (hydrocarbon) in the cylinder.
The aforesaid prior art (mentioned in Japanese Laid-Open Patent Application No. Hei 9 (1997) - 4499) aimed to overcome problems including the engine torque drop due to preignition caused by the overlapping of the electric discharging period after ignition and the fuel injection period in the non-combustion cylinder. Thus, this prior art does not solve the problem of overlapping of the fuel injection end timing and the ignition timing.